With a phased array antenna installed at base station, beamforming can enable a base station to provide better coverage for mobile stations that are operating at the edge of the cell. To enable the base station to form a beam on the transmit side pointing in the right direction for a specific mobile station, the base station needs to know the spatial channel information. As is known in the art, a covariance matrix can be obtained by correlating the signal vector received at the phased array. In Time Division Duplex (TDD) operation, this covariance matrix observed on the uplink can be used for transmit beamforming given that the time delay between receiving and transmitting is within the coherence time of the time-variant channel. However, in Frequency Division Duplex (FDD) operation the downlink covariance matrix is different from the one obtained on the uplink, so the beamforming design in FDD systems requires more attention.
One option is to operate the beamforming in closed-loop mode where the mobile station provides feedback to its base station. The mobile station can obtain the channel matrix using reference signals orthogonal amongst the base station transmit antennas and feedback the channel state information. However, this scheme requires signaling overhead (which consumes uplink resources), and such feedback suffers from quantization error and bit errors in the course of signaling back the channel state information.
Another option is to provide open-loop beamforming, which does not require feedback from the mobile station and therefore has the advantage of no signaling overhead. As is known in the art, when the frequency separation is relatively small with respect to the carrier frequency, the propagation paths between the uplink and downlink are highly correlated. Therefore, by processing the uplink spatial channel information, optimal or near-optimal beamforming can be generated on the downlink. One such technique is to form the beam using the angular spectrum, which is derived from the uplink and is unchanged between the frequency bands. However, this approach requires angle of arrival (AoA) estimation, which is computationally intensive.
The above operation only considers the propagation channel. However, in a practical communication system, the overall channel includes the internal signal path. In TDD mode, the accounting for both the propagation channel and the internal signal paths is addressed by array calibration. In FDD mode, calibration of the internal signal paths is still possible using a calibration transceiver in a way similar to what is done in TDD. However the antenna response on the two frequency bands varies, and the calibration of the antenna response variation is extremely hard if not impossible to accomplish. Therefore, the unequal antenna response on the uplink and downlink may have an adverse impact on the open-loop beamforming gain.
Therefore, it would be advantageous to have an approach to form beams in FDD mode without either feedback from mobile station or calibration on two carrier frequencies.